What is size reduction. Advantages and disadvantages of size reduction.

 

INTRODUCTION

Size reduction or comminution or Diminution or Pulverizations is the process of reducing large substances into smaller particles. Size reduction machines for large particle sizes (ie. particles above 40 mm), are known as crushers, while the particle sizes below this are processed by mills. Size Reduction is an important operation in many pharmaceutical applications. For pharmaceutical purpose, mono- size particles are best of use. For solid materials, grinding and cutting are used as a size reduction process, while emulsification or atomization for liquid material are used.

ADVANTAGES OF SIZE REDUCTION

1. For effective mixing particles of uniform size are desirable.  As content uniformity is important in case of potent and low dose drugs formulations
2. Size reduction increase surface area of ​​material and this further improve rate of dissolution.
3. During compression of tablet, particle size should be small so that powder can easily flow into dies. 
4. Extraction of active constituent become effective if smaller particles are used.

 5. Drying will be effective when small sized granules or powders are used.

DISADVANTAGES OF SIZE REDUCTION

1. Thermolabile substances get decomposed during size reduction.  This is due to heat produced during milling

2. There are chances of contamination during milling and grinding.

OBJECTIVES OF SIZE REDUCTION

1. Increase surface area: Reduced size leads to increased surface area.  For example: The dissolution rate of the solid drug particles increases many times after the size reduction Micronized form of Griseofulvin, an antifungal drug, shows about five times better absorption.
2. Ease of Mixing Reducing the size of particles in a narrow range makes mixing easier.
  3. Stability of suspension.  The fine particle size in pharmaceutical suspensions decreases sedimentation rate .
4. Dosage form: Pharmaceutical capsules, insufflations (ie, inhaled powders directly into the lungs).  suppositories and ointments require the particle size to be less than 60 mm in size. 
5. Stability of emulsion: The stability of emulsion can be increased by decreasing size of oil globules .
6. Reduce Irritation: Ophthalmic preparations and preparation means for external application to skin should be free from course and gritty particles to avoid irritation .

7. Increase absorption:  The rate of drug absorption will be higher if the particle size will be small .
8. Appearance By reducing particle size, physically appearance of ointment, creams, paste can be cnhanced.

FACTORS AFFECTING SIZE REDUCTION

1. Hardness It is a surface property of the material.  The hardness of material is measured by a devised known as Moh's Scale.  The Moh scale is from 1 to 10. Material of hardness 1 to 3 are classified as soft (eg talc, waxes), 4 to 7 intermediate (Limestone and bauxite) and 8 to 10 hard (Quartz and diamond).  The harder the material the more difficult it is to reduce in size.

2. Toughness The toughness of a material is sometimes more important than the hardness.  A soft but tough material may present more problems in reducing size than a hard but brittle substance.  For example, it is difficult to break the rubber than a blackboard chalk stick .

3. Abrasiveness Abrasiveness is a property of hard materials.  During the grinding of abrasive substances, the final powder may be contaminated with more than 0.1 percent of the grinding mil's worn metal .

4. Stickiness Știckiness is a property which causes considerable difficulty in reducing the size because the material get adhere to the grinding surfaces or the  screen meshes may blogged.

5. Slipperiness It is the opposite of stickiness.  It can also lead to size reduction difficulties as the material acts as a lubricant and decreases the efficiency of grinding surfaces.

6. Softening temperature during size reduction process, sometimes heat is generated which can cause some substances to soften, and the temperature at which this occurs may be important.  Waxy substances, such as stearic acid, or drugs containing oils or fats are examples that may be affected some methods can be used to overcome this like cooling the mill, either by a water jacket or by passing a stream of air through the equipment.

  7. Moisture content The moisture content influences a number of properties that may affect size reduction, For example, hardness, toughness or viscosity.  In general the materials must be dry or moist and not merely moist.  Usually, less than 5 percent moisture is adequate if the substance is to be ground dry or more than 50 if it is being subjected to wet milling.

MECHANISMS OF SIZE REDUCTION

The size reduction mechanism differs according to the nature of the material and each drug may require separate treatment.  The fracture occurs preferably along lines of weakness Fresh surfaces can be created or existing cracks and fissures can be opened, the former require much more energy. 

Size reduction is energy-inefficient process, since only a small percentage of the energy supplied is used to subdivide the particles.  Much of the energy is spent in overcoming the friction and inertia of the machine parts, the friction between the particles and the deformation of the particles without breaking them.  This energy is released as heat .

the particles to be reduced may be subjected to one or the combination of three forces
1. Compression
2. Shear
3. Tension

compression is crushing force, shear is cutting force and tension is force that tend to elongate a particle  or pull it apart by increasing the compression force, the flaws (discontinuity or imperfection) in the unit particle structure increase to a critical value.  This is called yield value.

             Fig:-  stress-strain properties 

The figure shows that there is an increase in deformation with increased stress up to the yield value and then breakage occur.  Compression at any point along the line below the yield value, the material will go back and return to its original shape and this is called elastic deformation.  While compression above the yield value will result plastic deformation in which the substance break down and will not return to its original form after removing stress.